Optical gage for testing surface imperfections



May 25 1965 J. M. HOLEMAN 3,185,022

OPTICAL GAGE FOR TESTING SURFACE IMPERFECTIONS Filed June 15, 1961 2 Sheets-Sheet l May 25, 1965 J. M. HOLEMAN 3,185,022

OPTICAL GAGE FOR TESTING SURFACE IMPERFECTIONS Filed June 15, 1961 '2 Sheets-Sheet 2 fr? Ver? br Ifo/7H 7. /o/eman United States Patent O 3,185,022 OPTICAL GAGE EUR TESTING SURFACE IMPERFECTINS John M. Holeman, Schenectady, NY., assigner to General Electric Company, a corporation of New York Filed June 15, 1961, Ser. No. 117,412 1 Claim. (Cl. {iS-14) This invention relates to optical testing gages and, more particularly, to such a gage for determining the flatness of a surface or its conformity toward a predetermined contour.

A known method of testing the optical flatness of a surface has been by the use of light interference. In that test, an optical iiat is placed in contact with the surface to be measured and the adjoining surfaces of the two pieces are then viewed by reflected monochromatic light. If the test surfaces are not in true optical contact, interference fringes will be seen and the number of such fringes appearing in the area can be used to determine by how many waveleng-ths of light the surface of the piece being tested is out of true atness. The light interference test has several disadvantages in that it requires a highly finished surface to produce fringes, it requires a monochromatic light source, and it requires the use of a high quali-ty, expensive optical iiat. In addition, the light interference test is extremely sensitive, with the maximum deviation in atness that is easily measured being less than .0005 inch.

When measuring the correctness of the radius of a surface, such as on a lens, for example, it has been customary in the past merely to hold the lens with a metallic gage in contact with the surface and see if there is light visible between them. This test involves the danger that a higher polished glass surface, such as on a lens, may be damaged by contact with a metal.

Accordingly, it is the primary object of the present invention to provide an optical testing gage which obviates the disadvantages of the light interference test and the metallic gage test.

It is a further object of the present invention to provide an optical testing gage that may be made in various forms for checking either the planeness of a surface or the correctness of the curvature of a spherical surface.

It is a further object to provide such a testing gage that may be used to check any surface that will reliect light and does not require a highly finished surface on the piece oeing tested.

It is a still further object to provide an optical testing gage that does not require the services of a skilled technician to perform the test.

The foregoing objectives are achieved in accordance with this invention by providing an optical testing gage comprising a light transparent supporting member carrying a thin, opaque film having a plurality of apertures therein. The light transparent backing may be flat or may be formed with any desired contour. When the testing gage is placed on the surface whose iiatness or contour is to be examined, and light is directed onto the gage, the light will be reflected from the surface through the same apertures through which it was incident if the surface is in close contact with the gage. If the surface is not in close contact with the gage, some or all of the reected light will be intercepted by the opaque portions of the film and the apertures in that area will appear darker. Thus, in effect, a typographical map of the surface is produced in which imperfect areas appear dark and perfect areas appear light.

Further objects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, in which FIGURE 1 is a plan view of a portion of an optical 3,185,022 Patented May 25, 1965 ICC testing device constructed in accordance with the invention;

FIGURE 2 is a sectional view taken on the lines 2-2 of FIGURE 1;

FIGURE 3 is a fragmentary bottom view of the gage shown in FIGURE 1 with a portion broken away;

FIGURE 4 is a sectional view similar to FIGURE 2 showing the device in use on a perfect plane test piece;

FIGURE 5 is a sectional view showing the device of the invention being used on an imperfect plane test piece;

FIGURE 6 is a sectional view showing the device being used on an imperfect convex surface; and

FIGURE 7 is a sectional view showing the testing device being used on an imperfect concave surface.

FIGURES 1, 2, and 3 illustrate in general form the optical gaging device of the invention, which comprises the light transparent supporting member 10 carrying a thin layer of opaque material 11 in which a-re provided a plurality of apertures or holes 12. It will be appreciated that in the drawings, the testing device is greatly enlarged in order t0 show its construction most clearly. In actual practice, the light transparent supporting member 10 may conveniently be made of thin glass and the opaque layer 11 may be photographic emulsion less than 0.001 inch thick with the holes 12 all being of the order of 0.01 inch in diameter. Preferably, the holes 12 are arranged as illustrated in hexagonal arrays spaced so that the distance separating any pai-r of adjacent holes is substantially equal to the diameter of the holes.

The operation of the testing device of the invention can best be understood from FIGURE 4, where the gage is shown being used to test the atness of the upper surface of a test piece 13. The lower surface of the transparent base 10 of the gage is, of course, of the desired llatness against which the test piece is to be measured. In the case illustrated, the surface of the test piece 13 is flat and rays 14 from a light source (not shown) incident on the surface being tested through the holes 12 are all reilected at the same angle from the flat surface and pass out through the same holes through which they entered. Therefore, if the reflected rays 15 are viewed by an observer, all of the holes 12 appear evenly illuminated, which indicates that the surface of the test piece is substantially flat.

FIGURE 5 illustrates the testing gage of the invention being used to check the surface of a test piece 16 having a dimple or depression 16a in the surface. It is seen that the incident light 14 entering through the hole 12a onto a ilat portion of the surface is returned through the same hole. However, light incident through the holes 12b and 12C is reflected from the irregular surface of the dimple 16a and, instead of returning through the same holes, strikes an opaque portion between the holes and may even be reflected through an adjacent hole. Thus, an observer viewing the reflected light 15 would see a dark area on the testing device corresponding to the position of the defect 16a.

FIGURES 6 and 7, respectively, illustrate the optical testing device of the invention being applied to test a convex surface and a concave surface. The gage may be formed to the desired contour for checking other than flat surfaces by grinding the transparent base from a blank or by forming it to the desired contour by the well-known optical manufacturing technique called dropping The dropping technique comprises heating the glass to a plastic condition and then dropping it ont-o a lap or mold having the desired configuration.

In FIGURE 6, the transparent base 10a of the tes-ting device is formed to a shape complementary to that of the surface on a piece 17 to be tested, which has an irregularity or depression 17a therein. The operation of the gage is like that previously described in that light 14 is incident f j on'the surface tobe tested through the holes 12a and 12b formed in the opaque film lla. As illustrated, the light incident through the hole 12a is reflected froman 11nblemished surfacel ofthe testl piece and'returns'asreected Similarly, in FIGURE 7, a transport base 10b is formed` VVto test a concave surface on a test piece 18 having an imperfection 18aV in its surface.` As in the embodiments previously described, incident light 14 passes through the openings 12a and l'Zb'in the opaque area 11b Vand it is reflected from the surface to be tested. Lightlincident through the hole 12a vis reilected back through the same hole through which it entered, whereas light incident` through the hole 12b is reflected from the irregular surface of the imperfection 18a-and does not. return through` the hole 12b. An observer viewing the retlected light kl5 would once again'see a dark spot on the testing gage in a position thatkv corresponds to the imperfection 18a on the test piece.

It is now apparent that the optical testing device of the invention fullls the* stated objectives and provides a relatively inexpensive yet sensitive testing device. It is also apparent that various modifications in construction and arrangement and size ofholes may be made byone 4 rf skilled in the art. Therefore, the invention is to be defined only by the scope of the appended claim.

What I claim as new and desire to secure by Letters Patent of the United States is:

An opticaltesting gage comprising a light transparent supporting member formed to a shape complementary to that of the general conguration but not the discrete irnperfections of a surface to be'1tested,and an opaque surface carried by said supporting member and having a plurality of regularly spaced light ltransmitting generally circular apertures formed therein, with adjacent apertures being separated by a distance substantially equal to their diameters and with suchv diameters being substantially equal whereby upon the gage being positioned over a surface to be tested andlight being directed through the apertures, light striking unblernished portionsV of thel test surface will be reflected back out throughthe apertures, and light striking imperfections inthe ltest surface Will be occluded by theopaque portions ofthe gage.

References Cited by the Examiner UNITED STATES PATENTS 1,478,536' 12/23 Servais 88-14 2,475,490 7/491 Gelardin 88-14 2,559,915 7/151 rGaebel t 88-14 2,657,611 ll/53 BorthY 3 3-174 2,867,149' l/59 Goddard 88-14 JEWELL H. PEDERSEN, i Primary Examiner.

Ell/HL G. ANDERSON, Examiner.` 

